ME : Electronics and Instrumentation Lab 1: Basic Resistor Circuits and DC Power

Transcription

1 ME : Electronics and Instrumentation Lab 1: Basic Resistor Circuits and DC Power Louis L. Whitcomb and Kyle B. Reed Department of Mechanical Engineering The Johns Hopkins University Spring 2009 Welcome to Electronics and Instrumentation, Lab 1. For this course s laboratory assignments you may work with up to one partner. Groups of three are not permitted. Write up your report independently: Your lab report should use the attached cover page. Lab grading is anonymous. You will receive your personal secret code during the first lab. Pease submit you lab reports with your secret student code instead of your name. The format of your lab report should parallel this handout: every section number and title should be duplicated in your report, and the results from all labeled items should be included. Equations should be written CLEARLY AND COMPLETELY. Circuit diagrams and other figures can be hand drawn, but must be neatly included in the document, and carefully labeled and referenced from the text of your writeup. Questions marked with a dagger ( ) can be completed at home; that said, some are worth doing in lab so that you can check to verify that your lab data is reasonable. You and your lab partner can share the same lab data that you collected together in lab. You should produce your final lab report independently (without your partner). Working with your colleagues to brainstorm and share problem solving methods is OK. You must work independently when you do your final write-up, however, without reference to any collective problem solving session notes i.e. your final write-up has to come from your brain, your data, your lecture notes, and your textbook, but not by copying results from group sessions or other sources. Apparatuses DC power supply (Figure 1(A)), multimeter (Figure 1(B,C)), leads, breadboard (Figure 3). Components 3 1K-10KΩ resistors, 1K-10KΩ potentiometers. Overview The purpose of this lab is to learn how to use a breadboard for simple resistor circuits, how to operate a typical DC power supply, and how to use one of the most basic electronics instruments, a multimeter. Before you begin, briefly familiarize yourself with the manuals of the multimeter and DC supply. c 2009 Noah J. Cowan, Louis L. Whitcomb, and Kyle B. Reed do not reproduce without permission. 1

2 1 Pre-Lab Exercises Questions from Chapter 1 of [1]

3 (A) (B) (C) Figure 1: (A) Tektronix PS280 Power Supply. (B) Tektronix DMM254 multimeter and (C) Fluke 170 Series multimeter; please refer to the corresponding manual for details of operation. 2 DC Power Supply and Multimeter For these questions, set your multimeter to DC volts mode (check the users manual if you are unsure how to do this). Make sure the Tektronix PS280 DC supply is turned off, and that there are no wires hooked up to it. Turn all voltage knobs to zero (completely CCW), and all current knobs to about 9 o clock on the dial (pointer on knob pointing left). 1. Put the power supply in Indep. mode by making sure both tracking control buttons are out. (Refer to the power supply manual.) Connect the +/ (red/black) leads of the multimeter to the +/ terminals, respectively, of the rightmost variable output. Now, turn on the power supply. Measure and note what happens when you do each of the following: (a) Adjust the voltage knob for the leftmost variable output. What happens and why? (b) Adjust the voltage knob for the rightmost variable output. What happens and why? (c) Now adjust either current knob. What happens and why? (d) Draw a circuit diagram that shows the two variable outputs as voltage sources, and show how they are interconnected with the multimeter. 2. Return the voltage knobs to the CCW position, and the current knobs to 9 o clock. Now, put the power supply in Parallel mode by making sure both tracking control buttons are in. (Again, look this mode up in the manual.) Repeat 1a 1d. 3. Repeat 1a 1d for Series mode. 3

4 3 Simple Resistor Networks: Series and Parallel The TA will provide you with three resistors in the 1,000Ω to 10,000Ω range. Resistors are color-coded (Figure 2) to indicate the resistor s resistance value and precision. One of the lab partners should hand in the resistors with her/his lab tape the resistors to the front page of the lab. Most of the resistors we will use in this class have a 0.25-watt power rating. 3.1 Reading the Resistor Color Code Figure 2: Resister color code (from dzierba/honorsphysics). 1. Note for your lab writeup the resistance and precision indicated by the color code of three resistors call them R 1, R 2, and R Set your multimeter to measure resistance. Your multimeter measures voltage, current, and resistance with varying accuracy see the manual. What is the specified accuracy of resistance measurement for this instrument? 3. Measure and note the resistance of R 1, R 2, and R Compute and note the error (in percent) between the measured resistance values and the values indicated by the color code. Are your resistors within spec? 3.2 Ohm s Law All circuits must be built using the breadboard, and you must make neat and tidy breadboard circuits. Where noted, you must have the TA sign off on your breadboard; failure to do so will result in lost points. Electronics is an art and sloppy work makes for difficult debugging, even for simple circuits! First, turn your PS280 DC Power Supply OFF. Using a banana-to-banana test lead, connect chassis ground to the negative terminal of one of the variable supplies of the PS280 DC power supply. Use banana plugs to bring power ( + ) and ground ( ) to your breadboard terminals, using a thoughtful color choice (e.g. a red lead to the red terminal for + and a black lead to the black terminal for ) from this supply. A typical breadboard similar to those in this lab is shown in Figure 3. Your TA will have an example breadboard set up, and will discuss some best-practices of using breadboards. 4

5 Note that the terminals of the breadboard are not internally connected to any of the pins until you make that connection. So, using short, neat leads, bring power and ground from the terminals to the red and blue bus lines on one side of the board. Figure 3: A breadboard (image downloaded from Wikipedia). 1. Let your instructor inspect your breadboard at this stage. Note in your lab report whether or not your instructor signed off. 2. Your Power Supply should still be OFF! Create a simple circuit on the breadboard with one resistor (R 1 from above) in series with your DC supply. When you construct your circuit, give yourself a place to insert your multimeter in series with the resistor. Again, your circuit should be neat and tidy. Put the multimeter in DC current mode, and insert it into the circuit. Let your instructor inspect your breadboard at this stage. Note in your lab report whether or not your instructor signed off. Do not go on to the next step unless you have the TA s approval! 3. In this step, you are going to collect multiple voltage current pairs for each of your three resistors, so that you can test Ohm s law. by Turn on the DC supply. For one resistor (R 1 ) and each voltage (2.5V, 5V, 7.5V, 10V) in the table, repeat the following procedure: (a) With the multimeter completely disconnected from the circuit, adjust the variable supply to be approximately equal to the specified voltage for V s. Right now, ensure that the multimeter is in DC volts mode, and not DC current mode or you will blow a fuse in the next step! In DC voltage mode measure and note the voltage using your multimeter. (b) Disconnect the multimeter. Put it in DC current mode, and insert it into the circuit in series with the resistor. Measure and note the current. Now, repeat (a) and (b) with each voltage to fill out the table below. All that you need to report for this question is the above table, so duplicate this table in your report and be consistent with significant digits. 4. Create an XY plot that shows all four data points and two distinct lines. The X-axis should be current (Amps). The Y-axis should be voltage (Volts). Plot the raw data points. On the same graph, plot the line which is the least-squares best-fit linear curve of the form y = m x (not y = m x + b) i.e. the Y-intercept should be zero Volts. Describe how you computed the best fit solution. 5

6 Nominal Measured Measured Current Voltage (V) Voltage (V) through R 1 (ma) Table 1: Data to verify Ohm s Law. 3.3 Series Resistance and Voltage Divider 1. Put all three resistors in series on a breadboard. Note the individual resistor values. Use the multimeter to measure (and note) the total resistance. (Your circuit should NOT be hooked to the power supply rails at this point.) R 1 R 2 R 3 Get your TA to sign off on your breadboard. Note in report whether or not your TA signed off on your breadboard. 2. Using the individually measured resistances (not the nominally rated resistances), compute and note the theoretical value of the total resistance of the three resistors in series, and compare to your measurement (provide the % error). 3. Choose two of your resistors; hook them up in series on the breadboard with a 10V supply. Draw a picture of your circuit, noting the individual resistor values and their positions. It is expected by this point that you are using breadboarding best practices. +10V + V 1 + V 2 R 1 R 2 0V 4. Using your multimeter, measure and note the voltage drop across each individual resistor, V 1 and V 2, and the total voltage drop across the pair of resistors, V T. 5. Using the measured resistor values and your voltage measurements, verify (show your work) that V 1 = R 1 R 1 + R 2 V T. (1) 6. Now configure your multimeter to measure DC current. Use clip leads to insert the multimeter in series with your circuit. Make sure that you correctly insert the multimeter in series with the resistors; if you are not absolutely sure what you are doing, please ask the TA to double check your circuit or you may blow a fuse. Measure and note the actual current flowing through your circuit. 6

7 7. Using the measured resistor values and your current measurements, verify that V T = I T (R 1 + R 2 ), V 1 = I T R 1, and V 2 = I T R 2. (2) Report the % error between the left and right sides of the above three equations. 3.4 Parallel Resistors and Current Divider 1. Using the same pair of resistors as in the previous section, hook them up on your breadboard in parallel. Using your multimeter, measure (and note) the total resistance of the two resistors in parallel. R 2 R 1 2. Compute and note the theoretical value of the total resistance of the two resistors in parallel (based on measured individual resistance values). Compare this to your actual measurement. 3. Hook your circuit up to a 10V output of your power supply. Measure and note the supply voltage using the multimeter in DC voltage mode. 4. Configure your multimeter to measure DC current. One at a time, use clip leads to insert the multimeter in series with each of the resistors in your circuit. Measure and note the actual current flowing through each of the two resistors in your circuit. The circuit diagram below shows the configuration for measuring current through R 1. (Note: none of the currents and voltages are labeled.) 10V Ammmeter R 1 R 2 5. In your report, draw the two individual circuit diagrams for measuring I 1 or I 2 (see above circuit for measuring I 1 ), being sure to label the missing current and voltage values carefully. In theory, in what way does the multimeter (acting as an ammeter) affect the current? 6. Verify that V T = I 1 R 1, V T = I 2 R 2, and V T = I T R T (report % errors for each of these equations). 4 Potentiometers (Pots) Potentiometers consist of a 2-terminal resistance element provided with 1-terminal sliding contact. The contact can translate (linear potentiometer) or rotate (rotary potentiometer). By exciting the potentiometer with a fixed voltage, and measuring the voltage obtained on the sliding contact, you can measure displacements rotation or translation. 1. With the potentiometer shaft turned about halfway, using your multimeter (appropriately configured) measure and note each pair of resistances: R AC, R AB, and R BC. Verify that R AC = R AB +R BC. 7

8 (A) (B) Figure 4: (A) 3D perspective of a potentiometer (from Wikipedia). (B) Schematic of a pot. 2. Set the power supply to 10V (measure and record the actual value), and connect to terminals C (positive) and A (negative). With the potentiometer shaft turned fully counterclockwise, mark the potentiometer as indicated by your instructor. Using your multimeter (appropriately configured) measure and each pair of voltages: V AC, V AB, V BC. Verify that V AC = V AB + V BC. 3. Using your multimeter, measure the voltage V AB at four or more different positions of potentiometer shaft. Provide a circuit diagram of the circuit you are using. At each of four or more position, note the voltage V AB and the (estimated) angular displacement (in degrees) of the potentiometer from the initial position 4. Plot your data (voltage vs. θ). Explain your graphs. 5. Derive a theoretical mathematical function (no numbers at this point, please!), V AB = g(θ) that relates the angle of the potentiometer, θ, to the voltage across the AB terminal. The expression for g will have several parameters, including V AC (the input voltage), θ min, θ max, R AC, R min, R max. 6. Now, find the inverse of g, namely the function θ = f(v AB ). Again, do not plug in any numbers yet. 7. Show that this model matches the actual data that you collected. REMEMBER Use the cover sheet provided on the next page to hand in your lab. Please remember that you or your partner must turn in your resistors, taped to the front of your report. Remember to show your work. Please clean up your workstation to perfection when you are done. Get your station checked by your TA and have them sign the cover page for your lab indicating that your lab station is in great shape. References [1] Giorgio Rizzoni. Fundamentals of Electrical Engineering. McGraw Hill, New York,

9 ME Lab 1: Basic Resistor Circuits and DC Power My Secret Code : My Partner s Secret Code : Lab Date : Lab Station Number : Today s Date : You must have the T.A. inspect your lab station at the end of your lab session. T.A. Signature and Date : 9